A new chiral uranyl phosphonate framework consisting of achiral building units generated from ionothermal reaction: Structure and spectroscopy characterizations

Article abstract of DOI:10.1039/c5dt02667a

The ionothermal reactions of uranyl nitrate and 1,3-pbpH₄ (1,3-pbpH₄ = 1,3-phenylenebis(phosphonic acid) ligand in ionic liquids of [C₄mim][Dbp], [C₄mpyr][Br], and [Etpy][Br], respectively, afforded three new uranyl phosphonates, namely [C₄mim][(UO₂)₂(1,3-pbpH)(1,3-pbpH)·Hmim] (1), [UO₂(1,3-pbpH₂)H₂O·mpr] (2), and [Etpy][UO₂(1,3-pbpH₂)F] (3). Compound 1 exhibits a rare example of a chiral uranyl phosphonate 3D framework structure built from achiral building units of tetragonal bipyramidal uranium polyhedra and 1,3-pbp ligands. The structure adopts a network with channels extending along the b axis, which are filled with C₄mim⁺ and protonated 1-methylimidazole. In sharp contrast, compounds 2 & 3 both show pillared topology composed of uranyl pentagonal bipyramid polyhedra and phosphonate ligands. The layers are neutral in compound 2 with N-methylpyrrole molecules in the interlayer space, while compound 3 adopts anionic layer, and the charge is compensated with N-ethyl-pyridinium cations between the layers. Although compounds 1, 2, and 3 were synthesized under identical conditions with sole variation of the ionic liquid species, the resulting structures show a rich diversity in the local coordination environment of uranyl ions, the protonation of the phosphonate ligand, the conformation of ionic liquid ions, and the overall arrangement of the structure. All compounds were characterized by absorption, temperature dependent fluorescence, as well as infrared and Raman spectroscopies.

Full text of DOI:10.1039/c5dt02667a

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A new chiral uranyl phosphonate framework
consisting of achiral building units generated
from ionothermal reaction: structure and
spectroscopy characterizations†
Cite this: DOI: 10.1039/c5dt02667a
Tao Zheng,a,b Yang Gao,a,b Lanhua Chen,a,b Zhiyong Liu,a,b Juan Diwu,*a,b
Zhifang Chai,a,b Thomas E. Albrecht-Schmittc and Shuao Wang*a,b
The ionothermal reactions of uranyl nitrate and 1,3-pbpH4 (1,3-pbpH4 = 1,3-phenylenebis(phosphonic
acid) ligand in ionic liquids of [C4mim][Dbp], [C4mpyr][Br], and [Etpy][Br], respectively, afforded three new
uranyl phosphonates, namely [C4mim][(UO2)2(1,3-pbpH)(1,3-pbpH)·Hmim] (1), [UO2(1,3-pbpH2)H2O·mpr]
(2), and [Etpy][UO2(1,3-pbpH2)F] (3). Compound 1 exhibits a rare example of a chiral uranyl phosphonate
3D framework structure built from achiral building units of tetragonal bipyramidal uranium polyhedra and
1,3-pbp ligands. The structure adopts a network with channels extending along the b axis, which are filled
with C4mim+ and protonated 1-methylimidazole. In sharp contrast, compounds 2 & 3 both show pillared
topology composed of uranyl pentagonal bipyramid polyhedra and phosphonate ligands. The layers are
neutral in compound 2 with N-methylpyrrole molecules in the interlayer space, while compound 3
adopts anionic layer, and the charge is compensated with N-ethyl-pyridinium cations between the layers.
Although compounds 1, 2, and 3 were synthesized under identical conditions with sole variation of the
ionic liquid species, the resulting structures show a rich diversity in the local coordination environment of
uranyl ions, the protonation of the phosphonate ligand, the conformation of ionic liquid ions, and the
overall arrangement of the structure. All compounds were characterized by absorption, temperature
dependent fluorescence, as well as infrared and Raman spectroscopies.
Received 14th July 2015,
Accepted 14th September 2015
DOI: 10.1039/c5dt02667a
www.rsc.org/dalton
tion.2 The uranyl phosphonates are also promising materials
Introduction
in this field with nanotube3 or open framework structures that
Recently, several actinide based open framework structures are able to exchange with ions as Cs+ or Co(en)3
.
3+ 4
have been reported, showing promising applications in
On the other hand, chiral open framework materials are
nuclear waste separation and remediation processes. For widely used in enantioselective separation, asymmetric cataly-
instance, NDTB-1 is a thorium borate cationic network that sis etc. However, although chirality exists widely in the biologi-
−
could remove TcO4 from the simulated low level waste solu- cal system, it is quite rare in the inorganic and inorganic/
tion with high exchange capacity and selectivity.1 The most organic hybrid materials.5 R. E. Morris and X. Bu have sum-
recent example is a polycatenated uranyl carboxylate frame- marized that the synthetic approach towards chiral framework
work showing high resistance towards β and γ irradiation, as could be the utilization of chiral building blocks, chiral tem-
well as selective uptake ability of Cs+ ions from aqueous solu- plate, chiral solvent, and/or spontaneous crystallization from
achiral units, while the last one is the least common and
unpredictable route.5
Chiral uranyl phosphonates are exceedingly rare. A search
in the CCDC database reveals that there are only four hydro-
aSchool for Radiological and Interdisciplinary Sciences (RAD-X), Soochow University,
Jiangsu 215123, China. E-mail: diwujuan@suda.edu.cn, shuaowang@suda.edu.cn
bCollaborative Innovation Center of Radiation Medicine of Jiangsu Higher Education
thermally synthesized chiral uranyl phosphonate compounds
Institutions, Jiangsu 215123, China
cDepartment of Chemistry and Biochemistry, Florida State University, 95 Chieftain
reported, namely (UO2)3(bpH)2(bp)2·H2O, [C2H10N2][UO2(C1P2)],
[NEt4]K[(UO2)3(C1P2)2]·1.5H2O,and [NPr4][UO2(pmbH3)(pmbH2)-
H2O]·2H2O (bp = benzenephosphonic acid; C1P2 = methylene-
diphosphonic acid; pmb = p-xylenediphosphonic acid).4,6 The
structure of [C2H10N2][UO2(C1P2)] and [NEt4]K[(UO2)3(C1P2)2]·
1.5H2O are porous 3D framework. The (UO2)3(bpH)2(bp)2·H2O
Way, Tallahassee, Florida 32306, USA
†Electronic supplementary information (ESI) available: Synthesis of 1,3-pheny-
lenebis(phosphonic acid), CD spectra, EDS and X-ray crystallographic files in cif
format, for compound 1–3. CCDC 1412606, 1412603 and 1412604. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c5dt02667a
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Materials and methods
1,3-Phenylenebis(phosphonic acid) (1,3-pbpH4) was syn-
thesized following a similar procedure reported in the litera-
ture.10 The details of the synthesis is shown in the ESI.†
UO2(NO3)2·6H2O, solution of HF (40%), and ionic liquids were
obtained from commercial sources (J&K Chemical, Sinopharm
Chemical Reagent, and Lanzhou Greenchem ILs) and used as
received.
Syntheses of compound 1. A mixture of UO2(NO3)2·6H2O
(0.05 mmol, 0.0256 g), 1,3-pbpH4 (0.1 mmol, 0.0247 g), 1-butyl-
3-methylimidazolium dibutyl phosphate (1 mmol, 0.3451 g),
and 1 drop of HF (40%), was placed in a 15 mL Teflon-lined
Scheme 1 Phosphonate ligands and ionic liquids used in this work.
and [NPr4][UO2(pmbH3)(pmbH2)H2O]·2H2O possesses 1D stainless steel vessel and heated at 120 °C for 120 h, and
tubular and chain structure, respectively. All these structures then cooled to room temperature. IR (KBr, cm−1): 3450(s,b),
were constructed from achiral units.
3154(m), 3122(m), 3087(w), 2959(m), 2924(m), 2854(m),
Introduced by Morris et al., ionothermal synthesis has 1625(m), 1589(w), 1572(m), 1550(w), 1466(m), 1391(m),
opened a new route for preparation of functional materials, 1340(w), 1281(m), 1251(m), 1172(s), 1146(s), 1105(vs), 1075(vs),
owing to the numerous choice of cations and anions for ionic 1030(s), 982(vs), 909(s), 821(w), 800(m), 751(m), 698(m),
liquids (ILs), which could be either hydrophobic or hydro- 676(m), 630(w), 623(w), 574(s), 547(s), 531(s), 479(w), 439(m).
philic. In search of the chiral compound syntheses, chirality Raman (cm−1): 1585(m), 1539(w), 1459(w), 1414(w), 1385(w),
inducted ionothermal synthesis route has been demonstrated 1342(w), 1179(w), 1091(w), 1015(m), 990(m), 820(s), 677(m),
to be non-trivial.7 The ionothermal method has been just 411(m), 212(s), 175(m), 108(s), 83(s).
applied in the synthesis of uranyl phosphonate system
Syntheses of compound 2. Yellow rhombic crystals of
reported by Parker et al.8 and Zheng et al.9 originated from compound 2 were collected following the same process as
their importance in the spent nuclear fuel reprocess, especially compound 1, except 1-butyl-3-methylimidazolium dibutyl
the recent utilization of ionic liquid for solvent extraction phosphate was replaced by N-butyl-N-methylpyrrolidinium
process to replace organic solvent originated from its advan- bromide (1 mmol, 0.2902 g). IR (KBr, cm−1): 3450(vs,b),
tages such as low vapor pressure and nonflamability.
3164(w), 3063(w), 2991(w), 1625(m), 1592(m), 1473(w),
We report here a novel chiral uranyl phosphonate frame- 1403(m), 1255(s), 1209(s), 1189(s), 1169(vs), 1123(vs), 1102(s),
work structure synthesized ionothemally as well as two other 1078(s), 996(w), 942(m), 918(s), 837(s), 733(w), 699(m), 678(w),
achiral layered compounds, namely [C4mim][(UO2)2(1,3-pbpH)- 650(w), 621(w), 552(s), 522(s), 502(s), 473(m), 442(w), 419(w).
(1,3-pbpH)·Hmim] (1), [UO2(1,3-pbpH2)H2O·mpr] (2), and Raman (cm−1): 1590(w), 1454(w), 1400(w), 1346(w), 1266(w),
[Etpy][UO2(1,3-pbpH2)F] (3). (1,3-pbpH4 = 1,3-Phenylenebis- 1205(m), 1177(m), 1118(w), 1080(w), 1021(w), 992(m), 847(s),
(phosphonic acid); [C4mim][Dbp]
imidazolium dibutyl phosphate; [C4mpyr][Br] = N-butyl-N-
methylpyrrolidinium bromide; [Etpy][Br] = N-ethylpyridinium were obtained following the same process as compound 1,
bromide; mpr N-methylpyrrole; Hmim protonated except 1-butyl-3-methylimidazolium dibutyl phosphate was
=
1-butyl-3-methyl- 679(m), 348(w), 273(m), 200(m), 135(s), 93(s).
Syntheses of compound 3. Micro-crystals of compound 3
=
=
1-methylimidazole) (Scheme 1). The spectroscopic properties replaced by N-ethylpyridinium bromide (1 mmol, 0.1898 g).
of those compounds were studied using room temperature IR (KBr, cm−1): 3450(s,b), 3133(w), 3063(w), 2925(w), 2854(w),
UV-Vis absorption, temperature dependent fluorescence, IR 1634(m), 1590(w), 1488(m), 1401(m), 1317(w), 1252(s), 1176(s),
and Raman spectroscopies. Compound 1 has shown preferred 1160(vs), 1120(vs), 1109(vs), 1097(s), 1080(vs), 1056(vs), 998(w),
uptake of Cs+ to Sr2+ from aqueous solutions. As an important 984(w), 933(s), 908(vs), 824(m), 796(m), 784(m), 704(m),
message, here we have shown that the structures generated 685(m), 594(m), 577(m), 556(vs), 530(m), 512(m), 472(s),
from ionothermal method deviates sharply from those of the 461(m), 409(m). Raman (cm−1): 1629(w), 1585(w), 1444(w),
traditional hydrothermal reaction for the uranyl system, which 1393(w), 1149(m), 1116(m), 1074(w), 1025(m), 986(m),
is similar to the observation reported in our previous work and 824(s), 761(w), 679(w), 645(w), 490(w), 411(w), 314(w), 261(m),
by Parker et al.8,9
146(s), 83(s).
Crystallographic studies
Experimental section
Single crystals of compounds 1–3 were mounted on Cryoloops
Caution! All studies and operations were conducted in an author- with paratone oil and optically aligned on a Bruker D8-Venture
ized laboratory to investigate actinide elements. Standard pre- single crystal X-ray diffractometer equipped with a high-resolu-
cautions should be performed for handling radioactive materials, tion digital camera. The diffraction data were collected using a
as all uranium compounds used in these studies and operations CMOS detector at room temperature with a Turbo X-ray source
contained depleted uranium.
(Mo-Kα radiation, λ = 0.71073 Å, 50 kV/50 mA power). The data
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was reduced by SAINT package of Bruker and the structures of was racemic conglomeration with one major enantiomer. This
all compounds were solved by the direct method and refined is consistent with the solid Circular Dichroism (CD) spectrum
on F2 by full-matrix least-squares methods using SHELXTL11 collected from a bulk sample of compound 1, whose signal is
(2014 version) and all the non-hydrogen atoms were refined originated from the excess of the major enantiomer (Fig. S3 in
2+
anisotropically. The protons are difficult to locate since their the ESI†). The asymmetric unit contains two UO2 cations,
electron densities are too low, thus all hydrogen atoms except two 1,3-pbp ligands, one protonated 1-methylimidazole cation
those attached to water molecules were put in calculated posi- and one 1-butyl-3-methylimidazolium cation, which are all
2+
tions. The hydrogen atoms of disordered IL cations were not located at general positions. As shown in Fig. 1, both UO2
added.
cation adopts tetragonal bipyramidal coordination geometry,
forming UO6 units. The CPO3 moieties from the phosphonate
ligands adopt two coordination modes of A and B with the
uranyl centers graphed in Scheme 2. The four oxygen atoms
located in the equatorial plane are O3, O4A, O7A, and O11B
for U1 and O6, O8, O2C, and O10D for U2, respectively, which
are from four different 1,3-pbp ligands (symmetric codes for 3:
A: x + 1/2, −y + 3/2, −z; B: x, y, z − 1; C: −x + 3/2, −y + 2, z +
1/2; D: −x + 3/2, −y + 2, z − 1/2.). The UuO and U–O bond
lengths are in the range of 1.767(6)–1.776(6) Å and 2.276(9)–
Absorption, fluorescence, infrared and raman spectroscopy
UV-Vis absorption spectroscopy were measured from single
crystals of the compounds from 200 to 800 nm, using the
Craic Technologies microspectrophotometer. Under the
365 nm irradiation, fluorescence spectra was recorded from
400 nm to 700 nm at 293, 273, 223, 173, 123, and 93 K,
respectively. The room temperature infrared spectra were
measured (4000–400 cm−1) on a Bruker VERTX 70 FT-IR instru-
ment in transmittance mode. Raman spectra were collected in
the range of 50–2000 cm−1 with crystals placed on quartz
slides without oil. Circular dichroism spectroscopy (CD) was
performed on Circular dichroism spectrometer Model 410
(Biomedical, Inc.).
Results and discussion
Structure descriptions
Crystal structure of compound [C4mim][(UO2)2(1,3-pbpH)-
(1,3-pbpH)·Hmim] (1). Compound 1 crystallizes in a chiral
orthorhombic space group P212121 (Table 1). The structure was
refined as an inversion twin, with a Flack parameter of
Fig. 1 Building unit of compound 1 with atomic labeling scheme at
0.193(10), indicating that the crystal used for X-ray diffraction 50% probability, with all hydrogen omitted for clarity.
Table 1 Crystallography data and refinement details for 1–3
1
2
3
Formula
M
CCDC no.
Crystal system
Space group
a/Å
b/Å
c/Å
C24H32N4O16P4U2
1232.48
1412606
Orthorhombic
P212121
9.767(1)
19.132(1)
19.139(1)
90
90
90
3576.3(3)
4
2.289
2304
9.298
1.052
0.0329, 0.0551
0.0481, 0.0587
0.690, −0.931
C17H12NO18P4U2
1120.23
1412603
Monoclinic
I2/c
17.691(1)
9.241(1)
19.823(1)
90
114.697(6)
90
2944.2(4)
4
2.523
2044
11.283
1.032
0.0224, 0.0755
0.0294, 0.1035
1.594, −2.136
C13H16FNO8P2U
633.24
1412604
Triclinic
ˉ
P1
9.719(1)
10.379(1)
10.464(1)
110.067(2)
103.971(2)
105.105(2)
891.3(1)
2
2.360
592
9.337
1.203
α/°
β/°
γ/°
V/Å3
Z
ρcalcd/g cm−3
F(000)
μ(Mo-Kα)/mm−1
GooF on F2
R1, wR2 [I > 2σ(I)]
R1, wR2 (all data)a
(Δρ)max, (Δρ)min/e Å−3
0.0719, 0.2064
0.0788, 0.2090
5.654, −7.156
a R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 = [∑w(Fo2 − Fc2)2/∑w(Fo2)2]1/2
.
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Scheme 2 Coordination modes of the phosphonate group.
Table 2 Selected bond distances (Å) for compound 1
U1–O13
U1–O14
U1–O4A
U1–O7A
U1–O3
U1–O11B
U2–O16
U2–O15
U2–O2C
U2–O10D
U2–O8
1.767(6)
1.770(6)
2.276(9)
2.291(9)
2.292(9)
2.297(9)
1.771(6)
1.776(6)
2.280(9)
2.285(9)
2.291(9)
2.290(9)
P1–O1
P1–O2
P1–O3
P2–O5
P2–O4
P2–O6
P3–O8
P3–O7
P3–O9
P4–O10
P4–O12
P4–O11
1.516(9)
1.52(1)
1.53(1)
1.509(9)
1.51(1)
1.54(1)
1.51(1)
1.51(1)
1.54(1)
1.51(1)
1.511(9)
1.52(1)
U2–O6
Symmetric codes for 1: A: x + 1/2, −y + 3/2, −z; B: x, y, z − 1; C:
−x + 3/2, −y + 2, z + 1/2; D: −x + 3/2, −y + 2, z − 1/2.
2.297(9) Å, respectively (Table 2), which are in agreement with
published bond lengths. Each 1,3-pbp ligand is coordinated to
four uranyl centers since each CPO3 moiety is bonded to two
uranyl centers. In this manner, the UO6 units are linked along
b axis by the CPO3 tetrahedron, yielding a chiral 1D uranyl
phosphonate chain structure with 21 screw axis, and then
further connected by phenyl rings of 1,3-pbp ligands, resulting
in a chiral 3D structure with two other 21 screw axis along a
and c directions. The channels along the b axis are filled with
C4mim+ cations (Fig. 2). Hydrogen atoms were determined by
analyzing the coordination environment of oxygen atoms and
hydrogen bonds were found in O12–H5–O5 and O1–H9–O9,
with the distance between oxygen and hydrogen atoms being
1.708 Å and 1.757 Å, respectively. It is noteworthy that such
chiral structure built from achiral units are exceedingly rare in
the uranyl compound, especially for framework structures,
since the formation of such compound is arbitrary.4
Fig. 2 A chiral chain of UO7 units and CPO3 tetrahedra along b axis (a)
and the 3D structure was achieved by chiral chains further linked by
organic moiety of 1,3-pbp ligand along a and c axis (b and c) of com-
pound 1. UO6 unit is represented by yellow tetragonal bipyramid, and
CPO3 is purple tetrahedron.
2+
C2/c (Table 1), and the asymmetric unit contains one UO2
cation, one 1,3-pbp ligand, one coordinated water molecule,
and N-methylpyrrole with half occupancy (Fig. 3). UO22+ cation
adopts pentagonal bipyramid coordination geometry with five
oxygen atoms located in the equatorial plane being from four
Since the C4mim+ cations filled in the voids are potentially
capable to be replaced, an initial ion exchange experiment was
conducted by soaking crystals of 1 in Cs+ and Sr2+ solutions.
After this, the crystals were washed with distilled water and
characterized with EDS (Energy Dispersive Spectroscopy) to
determine the amount of Cs+ and Sr2+ sorbed by the crystals.
The EDS results show that the uptake of Cs+ is higher than
Sr2+, under similar conditions (Fig. S4 and S5†), suggesting a
decent exchange selectivity towards Cs+ possessed by com-
pound 1. The mechanism accounting for such an unexpected
selectivity is under further investigation.
Crystal structure of compound [UO2(1,3-pbpH2)H2O·mpr]
(2). Unlike compound 1, compound 2 is a two-dimensional
Fig. 3 Building unit of compound 2 with atomic labeling scheme at
50% probability, with all hydrogen atoms and N-methylpyrrole omitted
layered structure that crystallizes in monoclinic space group for clarity.
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CPO3 moieties, and the fifth one is occupied by the water
molecule (O1, O2A, O4C, O5B and O1w, symmetric codes for 2:
A: −x + 1/2, y − 1/2, −z; B: x + 1/2, −y + 1/2, z; C: −x, −y, −z.).
In contrast to 1, the CPO3 moieties from 1,3-pbp ligand only
coordinate to the uranyl centers with Mode A (Scheme 2). The
layers can be best described as two layers of 1,3-pbp ligands
that are linked to each other through the tilted uranyl mono-
mers, together with the π–π stacking found in the structure for
adjacent phenyl ring of 1,3-pbp ligands, where the centroid–
centroid distance between two phenyl rings is 3.591 Å
(Fig. 4).12
Fig. 5 Building unit of compound 3 with atomic labeling scheme at
50% probability, with all hydrogen omitted for clarity.
The UuO and U–O bond lengths are in the range of
1.753(4)–1.758(4)
Å and 2.358(3)–2.399(4) Å, respectively
(Table 3), which agree with published bond lengths.3,6a,13 The
bond distances of P1–O3 and P2–O6 are 1.551(4) and
1.557(3) Å, therefore O3 and O6 should be protonated. Those
bond distances of the rest P and O in phosphonate groups are
in the range of 1.476(4)–1.486(4) Å, [P1–O2, 1.485(4) Å; P1–O1,
1.486(4) Å; P2–O5, 1.476(4) Å; P2–O4, 1.486(4) Å], suggesting
that those oxygen atoms are deprotonated. Thus, the layers
of compound 2 was concluded to be neutral, with disordered
N-methylpyrrole molecules filled in the interlayer space.
Crystal structure of compound [Etpy][UO2(1,3-pbpH2)F]
ˉ
(3). Compound 3 crystallizes in triclinic space group P1
2+
(Table 1), and the asymmetric unit contains one UO2 cation,
one 1,3-pbp ligand, one fluorine atom, and one N-ethyl-
pyridinium cation with half occupancy (Fig. 5). Similar with
2+
compound 2, UO2
cation adopts pentagonal bipyramid
coordination geometry, forming a UO7 unit, with five coordi-
nation sites located in the equatorial plane being O1, O2A, and
O4C from three CPO3 moieties and two fluorine atoms (sym-
metric codes for 3: A: −x + 2, −y + 1, −z; B: −x + 1, −y + 1, −z;
C: x, y + 1, z). It is surprising that the fluorine atom is incor-
porated into the structure, while first two compounds do not
contain any fluorine atoms, indicated from the EDS measure-
ments (ESI†). The uranyl polyhedra existed as a dimer by edge-
sharing of two fluorine ions, which is further connected by the
bridging 1,3-pbp ligand along a and b axis with bonding
modes of both A and C (Scheme 2), yielding a negatively
charged layered structure (Fig. 6). The N-ethyl-pyridinium
cations were found filling between the layers through hydrogen
network to construct the 3D structure. The UuO and U–O
bond lengths are in the range of 1.73(2)–1.80(1) Å and 2.32(1)–
2.37(1) Å, respectively (Table 4). U1–F1 bond is 2.34(1) Å,
which is in the normal range of 2.289(4)–2.394 (1) Å.14 The
bond distance of P1–O3 is 1.56(1) Å and P2–O6 is 1.55(2) Å,
suggesting the protonation of O3 and O6.
Fig. 4 2D structure of compound 2 viewed along c and b axis (top) and
(bottom). UO7 is represented by yellow pentagonal bipyramid, CPO3 is
purple tetrahedron.
Table 3 Selected bond distances (Å) for compound 2
U1–O8
U1–O7
U1–O2A
U1–O5B
U1–O1W
U1–O4C
U1–O1
1.753(4)
1.758(4)
2.358(3)
2.374(4)
2.378(4)
2.388(4)
2.399(4)
P1–O2
P1–O1
P1–O3
P2–O5
P2–O4
P2–O6
1.485(4)
1.486(4)
1.551(4)
1.476(4)
1.486(4)
1.551(3)
The structural deviation among compounds 1, 2, and 3
We have previously reported the structural control of uranyl
phosphonates under hydrothermal conditions via the alterna-
tion of pH, organic templates, and temperatures by analyzing
the coordination modes of uranyl to phosphonate ligands.6a
The results provide insight of how the protonation of phospho-
Symmetric codes for 1: A: −x + 1/2, y − 1/2, −z; B: x + 1/2, −y + 1/2, z;
C: −x, −y, −z.
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Table 5 Reaction condition and structure comparison of compounds
1–3
Comp.
Mode
ILs
D
G
C
1
2
3
A, B
A
A, C
[C4mim][Dbp]
[C4mpyr]Br
[Etpy]Br
3D
2D
2D
UO6
UO7
UO5F2
[C4mim]&[mim]
[mpr]
[Etpy]
G: Coordination geometry of U(VI); C: Cations or molecules filled in the
frameworks.
and 3 are both pillared structures, detailed structural compari-
son reveals that these structures differ in uranyl local coordi-
nation environment, protonation of the phosphonates,
structure dimension, and the ionic liquid configuration as
listed in Table 5. From the view of the local environment of
the uranyl unit, in compound 1, it is connected to four oxygen
atoms from four CPO3 moieties, while four oxygen atoms and
one water molecule are connected to the uranyl unit in 2, and
three oxygen atoms and two fluorine atoms in 3. The water
molecules are probably from the uranyl nitrate hexahydrate
starting materials, whereas fluorine atoms are from the HF
used in the reaction as mineralizing agent. The coordination
mode analysis of the CPO3 moiety shows that in both A and C
mode, the phosphonate ligands are protonated, whereas in B
mode, they are fully deprotonated. In mode A, two uranyl
atoms (in compound 1, and 2) or uranyl dimers (in compound
3) are bridged to give uranyl phosphonate chains. Modes C
was adopted for connecting uranyl with phosphonates to get
layer structures. The combination of mode A and B upgrades
the overall structural dimensionality observed in 1. This is con-
sistent with our previous study, which shows that by control-
ling protonation of the ligand, it is possible to obtain target
compound with expected dimensionality.
Fig. 6 2D structure of compound
(bottom) axis. UO7 is represented by yellow pentagonal bipyramid, CPO3
is purple tetrahedron, and fluorine atom is green ball.
3 viewed along c (top) and b
Table 4 Selected bond distances (Å) for compound 3
U1–O8
U1–O7
U1–O2A
U1–F1B
U1–F1
U1–O1
U1–O4C
1.73(2)
1.80(1)
2.32(1)
2.34(1)
2.34(1)
2.35(1)
2.37(1)
P1–O2
P1–O1
P1–O3
P2–O4
P2–O5
P2–O6
1.53(1)
1.53(1)
1.56(1)
1.52(2)
1.52(1)
1.55(2)
Symmetric codes for 2: A: −x + 2, −y + 1, −z; B: −x + 1, −y + 1, −z; C:
x, y + 1, z.
It is also quite interesting to find in compound 1 that the
cation of the ionic liquid undergoes an in situ decomposition
from 1-butyl-3-methylimidazolium to 1-methylimidazole, pro-
viding an alternate preparation of a desired cation initiated
with a more complex starting material.
nate would affect the bonding between uranyl centers and
phosphonate ligands, and correspondingly the overall struc-
ture dimensions. However in ionothermal synthesis, losing pH
as a parameter of reaction conditions, Parker et al. proposed
that the roles that the ligand, the flux method, as well as the
ionic liquid conformation played were of equally importance
in determining the crystal structure topology. Our previous
work of uranyl pmb (p-xylenediphosphonic acid) compounds
also demonstrated a large structural variation between com-
pounds obtained hydrothermally and ionothermally, as they
all show layered structure with cations of ionic liquid filled
between the layers of uranyl phosphonates.9
Here in this work, compounds 1 to 3 were synthesized
using the same 1,3-pbp ligands under ionothermal conditions
and solely varying the ionic liquid species. Besides the most
significant variation among the three compounds, where 1 is
an extremely rare sample of chiral uranyl phosphonate frame-
work with exchangeable cations from the ionic liquid, while 2
Spectroscopic properties
The UV-Vis absorption spectra of three compounds shown in
Fig. 7 all adopt two major peaks. The peaks near 320 nm
(313.6 for 1, 319.1 for 2, and 330.1 for 3) are resulted from the
ligand to metal charge transfer of the uranyl unit, owing to the
hybridization of the uranium 5f orbitals and the oxygen 2p
orbitals, while the peaks around 420 nm are originated from
the vibronically coupled charge transfer within the uranyl
unit.15 The relative intensity between the two peaks were
similar of compound 2 and 3, while the 420 nm peak is much
stronger in compound 1, probably owing to the higher sym-
metry of the uranyl centers.
The fluorescence spectra of the three compounds were col-
lected from 200 nm to 800 nm at temperatures of 293, 273,
223, 173, 123, and 93 K, respectively. The observed charge-
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Table 6 Peaks of fluorescence spectra for compound 1–3
Compounds
Peaks
1
2
3
508(m), 529(vs), 553(s), 579(m), 607(w), 639(vw)
498(m), 519 (vs), 542(s), 568(m), 595(w), 625(vw)
498(m), 519(vs), 542(s), 568(m), 595(w), 624 (vw)
transfer based emissions of yellow-green light in the range of
500–600 nm (Table 6) are mainly ascribed to the LUMO–
HOMO electronic transitions coupled with symmetric and
antisymmetric stretch vibrations of the nearly linear OuUuO
unit14f,15 as shown in Fig. 8, which are typical in uranyl com-
pounds.14a,14c,16 Notably, the intensities of fluorescence
spectra of compounds 1 and 3 increase with the decrease of
temperature, while the trend for compound 2 is reversed, likely
Fig. 7 UV-Vis spectra of compounds 1–3. The spectra are elevated for
clarity.
Fig. 8 Fluorescence spectra at different temperature of compound
1–3.
Fig. 9 IR and Raman spectra of compounds 1–3.
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due to the effect of fluorine coordination which provides a 81402628), the Science Foundation of Jiangsu Province
different thermal quenching process. For those uranyl units (BK20140007, BK20140303), a Project Funded by the Priority
with tetragonal bipyramid coordination geometry, they com- Academic Program Development of Jiangsu Higher Education
monly present weaker luminescence signals than the pentago- Institutions (PAPD), and “Young Thousand Talented Program”
nal or hexagonal bipyramidal geometry ones, attributed to the in China. Support for TEA-S was provided by the Chemical
strict central symmetry of the uranyl site. However, the lumine- Sciences, Geosciences, and Biosciences Division, Office of
scence signal for uranyl units in compound 1 with tetragonal Basic Energy Sciences, Office of Science, Heavy Elements
bipyramid coordination geometry is quite strong, which is Chemistry Program, U.S. Department of Energy, under Grant
likely due to the breakdown of strict central symmetry of DE-FG02-13ER16414.
uranyl units induced by overall chirality of the structure.
Infrared (IR) and Raman spectra were also recorded for all
three compounds (Fig. 9). For IR spectra, the peaks between
References
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In summary, three new uranyl phosphonate compounds have
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Acknowledgements
We are grateful for funding supported by National Science
Foundation of China (91326112, 21422704, 21471107,
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